THE ORIGIN AND EVOLUTION OF THE MOJAVE PROVINCE

The Mojave crustal province in southwestern Laurentia is distinctive for its evolved isotopic signature evident in all isotopic systems. However, the origin of the Mojave province’s isotopic signature, and the nature of the boundary between the Mojave province and the adjacent Yavapai province have been debated in the literature. U-Th-Pb geochronologic and Lu-Hf isotopic analyses of zircon from the oldest metasedimentary and plutonic rocks of the Mojave province provide insight into its genesis. Detrital zircon age populations in the Death Valley region are dominated by 1.85 Ga detritus, as well as 2.4-2.7 Ga grains. Metasedimentary samples from across the southern Mojave province show the same detrital zircon age populations. We suggest that together these metasedimentary rocks comprise a regionally extensive turbidite basin that we propose to call the Vishnu basin. The Vishnu basin extends the from the Transverse Ranges in California to central Arizona. Maximum depositional ages and cross cutting relationships indicate that deposition of the Vishnu basin proceeded from west to east between 1.79-1.74 Ga. Initial ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratios of zircon separated from plutonic rocks that intrude the Vishnu basin yield a wide range of εHf(t) values that vary systematically in time and space. 1.79-1.75 Ga plutons were derived from a lower crustal melt-source region that includes older Vishnu-like detritus and >1.8 Ga metaigneous crustal substrate. Granodiorites become more radiogenic in initial ¹⁷⁶Hf/¹⁷⁷Hf ratios from 1.79-1.73 Ma also get younger west to east, indicating an eastward migrating arc system with progressively increasing depleted mantle contributions to Mojave province magmatism. This trend culminates with εHf(t) values that range from 0 to +10 at ~1.73-1.7 Ga, synchronous with the onset of deformation and metamorphism throughout the province. The temporal and spatial isotopic variation in plutonic and detrital zircon in the Mojave province is markedly similar to those documented in the Phanerozoic Circum-Pacific accretionary orogenic system.